Now at least 10 years with sea ice at 2050-like levels yet polar bears are still abundant

We’ve hit the seasonal Arctic sea ice minimum for this year, called this morning by US NSIDC for 19th and 23rd of Septmeber: 4.59 mkm2, the same extent as 2008 and 2010. This is not a “ho-hum” year for polar bears: it means that since 2007, they have triumphed through 10 or 11 years1 with summer ice coverage below 5.0 mkm2 — levels that in 2007were expected to cause catastrophic declines in numbers.

The sea ice models used to support the addition of polar bears to the US Endangered Species List as ‘threatened’ with extinction suggested sea ice levels from 3-5 mkm2 would not occur unti mid-century, yet they dropped before the ink was dry on the 2007 USGS Reports (ACIA 2005; Hassol 2004; Holland et al. 2006; Solomon et al. 2007; Zhang and Walsh 2006).

The ice extent charts from the University of Bremen (below) show ice that’s 50% concentration or greater at the date of the seasonal minimum (19th September): what polar bear specialists define as preferred habitat (Amstrup et al. 2007).

Compare the minimum shown above to the coverage predicted for 2050 and to coverage at the minimum in 2012 (the NSIDC image is here):

Figure 3. Predicted sea ice changes (based on 2004 data) at 2020, 2050, and 2080 that were used in 2007 to predict a 66% decline in global polar bear numbers vs. an example of the sea ice extent reality experienced since 2007 (shown is 2012). See Crockford 2017 for details.

Footnote

Of these 12 years, two were only marginally above 5.0 mkm2 and one (2014) was within the margin of error for the threshold: 2013 (5.05) and 2014 (5.03). Only 2009, at 5.12 mkm2, was above that threshold. NSIDC states (Table 1) that values within 40,000 km2 are considered a tie, so the margin of error for these estimates can be assumed to be about 40,000 km2. That means the true value for the minimum at 2014 could be below 5.0 mkm2. Values used by the USGS were for 50% sea ice concentration, which would in any case be slightly below the 15% concentration value used by NSIDC: that fact means that 2013 was probably below 5.0 mkm2 as well.